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Concept · Mechanics
Kinematics & motion graphs
displacement vs distance (scalar/vector), velocity vs speed, acceleration & sign conventions; uniformly accelerated motion (v=u+at, s=ut+1/2at2, v2-u2=2as); free fall; v-t and x-t graph interpretation (slope vs area)
Switches you to Physics
Named traps
18 ways this goes wrong.
Each is a named misconception the questions are built to catch. Open one for the move that tempts, and the fix.
- Acceleration zero at apexAssumes acceleration becomes zero at the top of a vertical throw rather than remaining g downward.→
- Average velocity as endpoint meanComputes average velocity as (vinitial+vfinal)/2 for non-uniform acceleration where it does not apply.→
- Below-axis area sign ignoredAdds the area below the time axis on a v-t graph as positive distance when computing net displacement.→
- Constant velocity implies accelerationBelieves a moving object at constant velocity still has nonzero acceleration, or that zero velocity means zero acceleration.→
- Displacement vs distance confusedTreats total path length (distance) as the vector displacement, or returns displacement when distance is asked.→
- Heavier objects fall fasterAssumes a more massive object has greater free-fall acceleration, ignoring that g is mass-independent.→
- Inconsistent sign for gMixes up-positive and down-positive conventions within one problem, mis-signing g relative to displacement and velocity.→
- Instantaneous vs average velocityUses average velocity over an interval as if it were the instantaneous velocity at a point.→
- Negative acceleration means slowingAssumes negative acceleration always means decelerating, ignoring that it speeds up an object moving in the negative direction.→
- Path length used for sSubstitutes total distance travelled for s when the equation needs net displacement (e.g. a throw that returns down).→
- Return speed differs from launchBelieves the speed returning to launch height differs in magnitude from the launch speed, ignoring symmetry.→
- Up/down free-fall asymmetryTreats time to rise and time to fall (same height, no air resistance) as unequal, breaking the symmetry.→
- Velocity nonzero at top of throwThinks the velocity at the highest point of a vertical throw is nonzero rather than momentarily zero.→
- Velocity vs speed confusedReports average speed when average velocity (displacement/time) is required, ignoring direction reversals.→
- suvat used with changing accelerationApplies the constant-acceleration suvat equations to a situation where the acceleration is not constant.→
- v-t area/slope swappedUses the slope of a velocity-time graph for distance, or its area for acceleration, swapping the two operations.→
- v2=u2+2as mis-squaredForgets to square u or v, or drops the factor of 2, when using v2-u2=2as.→
- x-t value read as velocityReads the height of a position-time graph as the velocity instead of using its slope.→
Key formulas
No formula sheet is matched to this concept yet. Work it from first principles, and watch the traps.
Shaky underneath?
If the concept underneath still feels loose, patch it here before coming back to CSCA questions.
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